Permeable tubular membrane modules, sometimes called permeators, are used in a wide variety of fluid (gas or liquid) separations. In such operations, a feed stream is brought into contact with the surface of a semi-permeable membrane.
Membranes are thin barriers that allow preferential passage of certain components of a multi-component fluid mixture. Most membranes can be separated into two types: porous and nonporous. Porous membranes separate fluids based on molecular size and/or differential adsorption and diffusion rates. Gas separation membranes used in natural gas applications often have an asymmetric structure composed of a support structure that provides mechanical strength and a thin active layer that separates gases based on differences in solubility and diffusivity.
In a typical membrane separation process, a multi-component fluid is introduced into the feed side of a module that is separated into two compartments by the permeable membrane. The fluid stream flows along the surface of the membrane and the more permeable components of the fluid pass through the membrane barrier at a higher rate than those components of lower permeability. After contacting the membrane, the depleted feed stream, known as the “residue”, “retentate”, or “non-permeate”, is removed from contact with the membrane by a suitable outlet on the feed compartment side of the module. The fluid on the other side of the membrane, known as the “permeate”, is removed from contact with the membrane through a separate outlet. The permeate stream from the membrane may be referred to as being “enriched” in the permeable components relative to the concentration of the permeable components in the retentate stream. The retentate may also be referred to as being “depleted” of the more readily permeable components. While the permeate stream can represent the desired product, in most natural gas permeation processes the desired product is the retentate stream, and the permeate stream comprises contaminants such as CO2 or other acid gases.
Most prior art membrane modules include: 1) individual hollow fibers or membrane tubes, or bundles of fibers or membrane tubes, 2) membrane tubesheets in the form of solid bodies of suitable material for potting the opposite ends of the membrane tubes such that their internal bores, or lumens, communicate through the membrane tubesheets, 3) a pressure container formed by an elongated pressure vessel, and 4) a pair of opposite end heads or caps closing the opposite ends of the pressure vessel. The pressure vessel thus contains, protects, and supports the tubular membranes. The opposite membrane tubesheets with the membrane tubes extending therebetween are supported and sealed within the pressure container and interior manifolds or chambers are formed between the outer faces of the membrane tubesheets and the vessel's end caps through which communication is established between end cap ports and the lumens of the membrane tubes which open at the outer faces of the membrane tubesheets. The tubular membranes are typically made from polymeric materials and the pressure vessel is typically made from either polymeric materials (for low-pressure applications) or steel (for high-pressure applications).
One disadvantage of such prior art modules is that the tubesheet thickness and weight increase significantly as the pressure rating and/or module diameter increase. Because of this, in some applications the tubesheet can become very thick, thereby significantly reducing the module packing density and increasing the weight beyond practical limits. When attempts are made to produce a large-diameter module, the large amount of membrane tubesheet material that must be positioned around the ends of the membrane tubes can present handling problems in positioning the membrane tubesheet around the ends of the membrane tubes. During operations in which wide variations in temperature occur, the membrane tubes and the membrane tubesheets can expand and contract which can compromise the integrity of the seal between the membrane tubesheets and the housing container.
Although most prior art membranes are based on relatively flexible polymer materials, a new generation of high-performance membranes is being developed based on relatively rigid, inorganic materials, such as micro-porous ceramics. These new materials potentially have greater resistance to chemical attack, greater thermal stability, increased permeance rates, and increased selectivity compared with existing polymeric membrane materials. One drawback with using these new materials with the prior art membrane module designs is that the pressure vessel is made of a first material that has disposed inside membrane components made of a second material, and which materials undergo differential expansion relative to one another. This differential thermal expansion can compromise the integrity of internal seals and can lead to mechanical failure of the membrane materials. It would be a major advance if large-diameter membrane modules having elongated tubular membranes could be made which overcome or minimize the problems associated with sealing the permeate side from the feed side under fluctuating temperature conditions.